Servomechanism, particularly a vehicle speedometer or trip-recorder

ABSTRACT

A servomechanism, particularly a vehicle speedometer or triprecorder includes a controlled member, such as a speedometer needle or a trip-recorder scribe, as well as a servomotor and a drive gear driven by the servomotor. A servoamplifier has an output connected to the servomotor and a differential input. A command unit connected to the amplifier input furnishes thereto signals indicative of a commanded position for the controlled member. A feedback transducer furnishes to the amplifier input signals indicative of the position of the controlled member, and has a transducer input gear driven by the drive gear and defining therewith a first meshing region having a predetermined orientation relative to the axis of the transducer input gear. A driven gear driven by the transducer input gear and driving the controlled member, defines with the transducer input gear a second meshing region having a predetermined orientation relative to the axis of said transducer gear which is angularly offset from the orientation of the first meshing region by an angle of substantially less than 180*, and most advantageously, 0*.

United States Patent [1 1 Helmschrott [5 I SERVOMECHANISM, PARTICULARLY A 1 VEHICLE .SPEEDOMETER R TRIP-RECORDER [75 inventor: NorbertHelnrschrott,

Schwenningen, Germany [73] 'Assigneez' Kienzle Apparate GnibH,

' 'Villingen/Schwarzwald, Germany 22 Filed: :June23,197 2 [21] Appl. No.: 265,797

[30] Foreign Application Priority Data June 26,1971 Germany; 2131915 2 US. Cl. 318/61s,318/611 [51] Int.,Cl. G0b 5/01 [58] Field of Search 3l8/61l ,618

[56] References Cited 5 UNITED STATES PATENTS 2,386,367 10/1945 Taylon; 2,734,396 2/1956. Falk 2,9l.l,845 '11/1959, North 3,093,784 6/1963 341611 1,... 3,167,975 2/19 5 Durand 3,245,279 4/l966 Baker 3,283,231, 11/1966 Askew 3,400,212 9/1968 Plummer 318/618x July 23, 1974 1/1971 Fournier 318/618 x 5/1972 Plummer 318/6 18 X Primary Examiner-T. E. Lynch Attorney, Agent, or Firm'Michael S. Striker [57] ABSTRACT 4 A servomechanism, particularly a vehicle speedometer or trip-recorder includes a controlled membensuchas a speedometer needle or a trip-recorder scribe, as well as a servomotor and a drive gear driven by the servomotor. A servoamplifier has. an Output connected to the servomotor and a differential input. A command unit connected to the amplifier input furnishes thereto signals indicative of a commanded position for the controlled member.. A feedback transducer furnishes 5 to the amplifier input signals indicative of the position of the controlled member, and has a transducer input gear driven by the drive gear and defining therewith a first meshing region having a predetermined orientation relative to the axis of the'transducer input gear; A P

driven gear driven by the transducer input gear and driving the controlled member, defines with the transducer input gear a second meshing region having a predetermined orientation relative to the axis of said transducer gear which is angularly offset from the orientation of the first meshing region by an angle of substantially less than and most advantageously, 0.

- 10 Claims, 3 Drawing Figures tor transducer. i Still-more particularly,

. I 1 SERVOMECIIANISM, PARTICULARLY A VEHICLE SPEEDOMETER OR TRIPrRECORDER BACKGROUND .OF THE INVENTION The invention relates to servomechanisms, particularly vehicle speedometers and trip-recorders.

More particularly, the inventionrelates to vehicle speedometersand trip-recorders incorporating feedback compensation, i.e., wherein the command signal indicative of engine speed is applied'to the differential input of a servoamplifierto which is also applied a feedback signal from a position-responsive feedback transducer, and particularly an angular-position-responsive feedback transducer. Such'feedback transducer can for instance be a rotary potentiometer or a rotary capacithe invention relates to vehi- .Cle speedometers and the like, wherein the transducer input gear is located in the servomechanism gear train intermediate" the drive gearand the driven gear associated with the speedometer needle, trip-recorder scribe, or the-like. I

The problems associated with the design of vehicle speedometers and mp recorders are different from those prevailing in the design of most other measuring instruments andindicators. Specifically, very precise correspondence between vehicle speed and speedometer-needle deflectionis not feasibly attained except by the use of feedback compensationxnetw'orks, that is, a servomechanism whose controlled. member is the speedometer needle andwhose input signal is. alspeedproportional voltage. derived froma suitableAE. generator, for instance. The principlesof servomechanisms are of courseex'tremely well known.

It is common practicein such arrangements to drive the transducer gear off ofgone of the. gears in the gear trainbetween the; servomotorand. the controlled. member. It -is also, knownto incorporate the-transducer gear into the main gear-train .itself,that is, so that it serves as an intermediate, gear driven by. themotoranddriving the controlled. member.

A predictable difficulty resultingafrorhtheuse ofsucht gear trains in speedometer; servornechanismsandv the like is the 'phenomenonfof backlash. That is, the; play between meshing gear teeth results in; a: certain dead time when'the direction of drive. is reversed; This re-- sults in a substantial increaseinithepercentage-error of the reading actually provided by; the speedometer or= trip-recorder. It is the. backlashxfactor in particular. which motivates the use; of- 'thefeedback-transducer (potentiometer) gear as an intermediate gear driveneby the motor andgdriving the controlled member; In addi+ tion, a;permanentunidirectional biasing: force.- is cornmonly. .applied'to oneof the'final gears, particularly to the gear directly associatediwith the controlled memher. This unidirectional biasing force rwill preventibacklash, at. least in thesfinali gears of thetrain:

While backlash is thussubstantially. reducedi the biasing force creates new problems. Specifically, when bearing stress is particularly serious'jwhen, in the interest of reducing backlash to zero, a strong biasing force is utilized.

Clearly, it is-possible to deal with these bearing stresses by providing excellent bearings which are highly wear-resistant and which also permit very free turning despite such radial bearing stress. Ball bearings,

for instance, meet such requirements. However, ball bearings, and other high-quality bearings, are fairly expensive, and their proper installation increases the assembly cost, and increases the skill required of the worker who assembles the gear train.

SUMMARY or THE INVENTION It is accordingly a general object'of the invention to overcome such disadvantages of known vehicle speedometer and trip-recorder servomechanisms.

It is a more particular object to provide a speedome+ ter servomechanism whose mechanical parts are argear driven by the servomotorl A servoamplifier has an output connected to the servomotor andhas axdifferem tial input. A command unit connected to the differential amplifier input furnishes thereto signals indicative of a commanded: position for the speedometer-needle.

A feedback transducer furnishes to the amplifier input signals indicative of the actual pos itionof the speedometer needle, or trip-recorder scribe. The feedback transducer has atransducer input geardriven bythe drive. gear and defining therewith a first meshing region having'a certain orientation relative to the axisof the transducer input" gear. A driven gear is driven" by the transducer input gear and serves to drive the controlled member. The drivengeardefines with the transducer input gear a second meshingregion having a certain 1 orientationrelativeto theakis of said transducer-input gear which is angularly offset"from theorientation of the firstmeshing region by anangle of substantially less than 180. Advantageously, the angular offset between the first and secondmeshing regions is ,very small, and

when the angular offset is 0, the radial bearing stress can be all but completely eliminated. I

The novel features which are considered as characteristic for the invention are set forthin particular in the appended claims. The invention itself, however,

both' as to its constructionand its method of operation,

together with additional objects and advantages the gear-train is driven in one of its two directions,'.the

driving; torque appliedito the; transducer gear-will oppose, rather than-augment: the backlash-preventing abiasing torque applieditotheitransducerxgear, andiin such a manner that theresultantvforcezisrpredominantly ra dial. Such radial force. produces very significant" stresses on the bearings for 'the transducer gear; This thereof, willbe best understood from the following 'description ofzspeciflc embodimentswhenreadin connection with the accompanying drawingi BRIEFADESCRIPTION'OF THE DRAWING I FIG. 1 is a block diagram of aspeedometer/triprecorder servomechanism;

FIG. 2 illustrates the arrangement of gears in the servomechanismofFIG. l; and

It is a still more specific object to provide a speedo m-.

3 FIG. 3 illustrates the meshing between two particular gears of the servomechanism of FIG. 1.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS The block diagram of FIG. 1 provides an overall picture of the speedometer servomechanism according to the invention. The servomechanism concepts as such are of course not novel.

The arrangement can include either or both of two controlled members, namely a speedometer needle 12 and a trip-recorder scribe 11. Speedometer needle 12 is angularly displaceable over acalibrated scale 13, and scribe l 1 is mounted for slidinng movement on carriage l and is driven by an associated gear 9.

The controlled members 1 1, 12 are driven by a servomotor Shaving an output gear 16. The dotted lines in FIG. 1, in conventional manner, symbolize mechanical linkage.

The activating signal for servomotor 5 is applied'from the output of servoamplifier 4 via amplifier end stage 8. Amplifier 4, in conventional manner, has a differential input 3. Also in'conventional manner, the amplifier has a feedback network. 6' connected from the am pliifier output to the amplifier input. Furthermore, the servomechanism includes a command unit, comprising a tachometer generator 1 which produces pulses at a frequency corresponding to engine or wheel speed, and a frequency-to-voltage converter 2, which applies to the input of servoamplifier '4 voltage correspondingto engine or wheel speed. Finally, feedback transducer means 7 is mechanically connected with the controlled members 11, 12 and electrically connected with the input of servoamplifier 4. In known manner, feedback transducer 7 applies to the amplifier-input a voltage indicative of the actual. position of needle 12 and/or scribe l1. The feedback transducer .7 can be a rotary potentiometer, a rotary variable capacitor, or any other suitable type of transducer. The operation of the illustrated speedometer servomechanism is conventional.

The gear train of the illustrated speedometer servomechanism is shown in FIG. 2.

Servomotor 5 has an output shaft 15 which is rotatably mounted by a supporting wall 14. Provided on the end of output shaft 15 is an output gear 16. Rotary transducer 7 has an input shaft 23 likewise rotatably mounted by supporting wall 14. Input shaft 23 carries a transducer input gear 23.

A first intermediate gear 17 is rotatably mounted on axle 18, which in turn is supported by support wall 14. A drive gear 19 is likewise supported on axle 18 and rigidly, i.e.. non-rotatably, connected with first intermediate gear 17. Drive gear 19 receives torque from servomotor 5 and drives transducer input gear 23.

Transducer gear 23, inturn, drives a driven gear 20, which is also mounted on axle 18, but which is rotatable independently of the rotation of drive gear 19.

A second intermediate gear 21 is mounted for rotation on axle 18 and is rigidly connected with driven gear 20.

v Second intermediate gear 21, finally, drives indicator gear 22. In the drawing, and for simplicity, the speedometer needle 12 is shown as mounted directly on indicator gear 22. However, a different-number of intermediate gears may intervene between driven gear and the gear carrying or driving the indicator'member. Likewise, by an appropriate choice of gearing ratios,

4 the speedometer needle 12 might be directly associated with the driven gear 20 itself. Likewise, the triprecorder gear 9 might be the driven gear 20 itself. Many such modifications are possible. It should also be remembered that the gears employed may be of any type, for instance bevel gears, skew bevel gears, etc.

In the illustrated embodiment, the indicator gear 12 is biased by (schematically illustrated) biasing means 35 and permanently urged to turn in one of its two directions. Such biasing is provided in particular to eliminate backlash. As will be appreciated, the biasing force exerted by means 35 on gear 22 is communicated successively to gears 21, 20, 23 and 24, in that order, and also to some extent via gear 24 to gears 19, 17 and 16. If, to reduce the backlash effect to an absolute minimum, the force transmitted through the gears from bibiasing torque which is applied to gear 24 by gear 20 will be of a significant magnitude.

The great advantage of the invention lies in the great reduction, and even elimination of bearing stresses on transducer shaft 23. The following explanation is offered of how' such improvement is effected:

As mentioned, gear 20 always exerts a biasing torque on transducer gear 24. This force is alwaysin a particular direction (e.g., into the plane of the drawing sheet) regardless of the direction in which the gears turn. This is because of the unidirectional biasing action of biasing means 35.

Drive gear 19 also exerts a torque on transducer gear 24. However, the direction of this torque changes when the direction of gear rotationchanges. In one direction of motor rotation, gears 19, 20 will apply torque to gear 24 in the same direction, so that the torques add. In the opposite direction of motor rotation, gears 19, 20 will apply to gear 24 oppositely acting torques, and the torques will subtract. Most importantly, when the torques oppose, no net radial bearing stress results. This is because the region of meshing between gears 19, 24 is angularly displaced from the meshing region between'gears 20, 24 only by a small angle, preferably zero, and no net radial force results.

In the prior art, however, the angular displacement between the two meshing regions was always very great, most often In FIG. 2, reference characters PA indicate where in the prior art the force from drive gear 19 was conventionally applied. As will be appreciated, in one of the two directions of gear rotation, the prior-art torque applied to gear 24 at region PA would oppose the back-bias torque applied (as in FIG. 2) by gear 20. Accordingly, in the prior art, the opposing torques would result in a net radial force passing, for example, normal to the paper plane of FIG. 2. This is the bearing stress which the present invention can greatly reduce and even eliminate.

This radial bearing stress decreases as the angular offset between the first meshing region (gears 19, 24) and the second meshing region (gears 24, 20) decreases. Consequently it is most advantageous, although not absolutely necessary, to make this angular offset zero.

In the illustrated embodiment, the angular displacement between the first and second meshing regions is made zero aqrammusm. driven gear 20. solutely necessary that this be the case. The radial bearing stress can also be reduced to zero even if gears 19,

by making drivegear 19 coaxial to and of However, it is not abi v 20 are not'coaxial and not'of identical radius and toothing. It would be sufficient,for example, if gears 19, 20 were of different radius and non-coaxial but meshed with gear 24 at any given moment. Other modifications are possible. within the general framework of the inventive concept. For instance, the angular offset between meshing regions could be quite small, for instance 3 or 4, or even or 20, that is, not exactly zero. A great improvement will result providing only that the angular offset is very substantiallyless than 180, the most disadvantageous angle.

In particular, it is strongly emphasized that while a single transducer gear 24 is provided, the provision of two identical but distinct coaxial gears 24, each mesh- .ing with adifferent one of gears 19, 20, would clearly -be equivalent to theprovisionof-a single gear v24.

Moreover, such two coaxial gears 24 need not be even be of equal radius, providing that angularoffset between the first meshing region (gear 19, one of gears 24) and thesecond meshing region-(gear 20,-other of gears 24),is substantially less than 180, and most advantageously of all, 0. i

Without further analysis, the foregoingwill so, fully reveal the gist of the present invention that other's'can beapplyin'g current knowledge readily adapt it for various applications without omittingfeatures that, from the standpoint of prior. art, fairly constitute essential characteristics of thegeneric or specific aspects of this invention and, therefore 'such adaptations should and are intended to be comprehended within the meaning and range of equivalence of thefollowingclaims,

What is claimed as new and desired to be protected. by Letters Patent is set forth in the appended claims:

1. A servomechanism," particularly a vehicle speedometer or'trip-recorder, c'orriprising,fincombination, a

. controlled member; a servomotor; a drive gear driven by said servometer; a servoamplifier having an output connected to said servomotor and having a differential input; command means connected to said differential input for furnishing thereto signals indicative of a commanded position for said controlled member; feedback transducer means for furnishing to said differential input signals indicative of the position of said controlled member, said feedback transducer means having a rotating transducer input shaft which is to be pro tected from forces exerted upon said input, shaft in direction radially relative to said rotating input shaft in order to prevent damage to said feedback transducer means, and further having a transducer input gear mounted on said rotatingtransducerinput shaft for rotation therewith, said transducer input gear being driven by said drive gear and defining therewith a first meshing region having a predetermined orientation relative to the axis of saiid transducer input gear; and a driven gear driven by said transducer input gear and driving said controlled member, and defining with said transducer input gear a second meshing region having a predetermined orientation relative to the axis of said transducer input gear which is angularly offset from the orientation of said first meshing region by an angle of substantially less than whereby the combined forces exerted upon said transducer input gear by said drive gear and by said driven gearat said first and sec"- ond meshing regions produce a resultant force whichis predominantly circumferentially directed instead of being directed radiallyrelative to said rotating trans ducer input shaft, to thereby prevent damage to said feedback transducer means such as could resultfrom the exertion of excessive radially directed force upon said transducer input shaft. 1

2. A servomechanism as defined in claim 1, wherein said angle is approximately 0, whereby the combined forces exerted upon said transducer input gear by said drive gear and said driven gear at said first and second meshing regions produce a resultant force which is substantially exclusively circumferential whereby to exert upon said rotatingtransducer input shaft substantially zero radially directed force. e

3. A servomechanism as defined in claim 2, wherein said drive gear and said driven gear are coaxial. v

4.. A servomechanism as defined in claim 2, wherein said drive gear and said driven gear are coaxiaLof equal radius and identically toothed. g I

5. A servomechanism as defined in claim 1 and further including biasing means for counteracting backlash in the transmission of force by, said gears by permanently biasing said driven gear in one rotational direction, whereby when said drive gear and said transducer input gear drive said driven gear in the opposite rota 6 A servomechanism as defined in claim 3, wherein said angle is approximately 0, whereby said resultant force will be substantially entirely circumferential and will have substantially no radial. component.

7. A servomechanism as defined in claim 6, wherein said drive gear and said driven gear are coaxial.

8. A servomechanism as defined in claim 7, wherein said drive gear and said driven'gear are coaxial, of equal radius and identically toothed. v

9. A servomechanism as defined in'claim 2, and further including a first intennediate gear non-rotatably connected to and coaxial with said transducer input gear and driven by said drive motor, and a secondintermediate gear non-rotatably connected to and coaxial with said driven'gear and driving said controlled member.

10. A servomechanism as defined in claim 6, and further including a first intermediate gear non-rotatably connected to and coaxial with said transducer input gear and driven by said drive motor, and a second intermediate gear non-rotatably connected to and coaxial with said driven gear and driving said controlled member. 

1. A servomechanism, particularly a vehicle speedometer or triprecorder, comprising, in combination, a controlled member; a servomotor; a drive gear driven by said servometer; a servoamplifier having an output connected to said servomotor and having a differential input; command means connected to said differential input for furnishing thereto signals indicative of a commanded position for said controlled member; feedback transducer means for furnishing to said differential input signals indicative of the position of said controlled member, said feedback transducer means having a rotating transducer input shaft which is to be protected from forces exerted upon said input shaft in direction radially relative to said rotating input shaft in order to prevent damage to said feedback transducer means, and further having a transducer input gear mounted on said rotating transducer input shaft for rotation therewith, said transducer input gear being driven by said drive gear and defining therewith a first meshing region having a predetermined orientation relative to the axis of saiid transducer input gear; and a driven gear driven by said transducer input gear and driving said controlled member, and defining with said transducer input gear a second meshing region having a predetermined orientation relative to the axis of said transducer input gear which is angularly offset from the orientation of said first meshing region by an angle of substantially less than 180*, whereby the combined forces exerted upon said transducer input gear by said drive gear and by said driven gear at said first and second meshing regions produce a resultant force which is predominantly circumferentially directed instead of being directed radially relative to said rotating transducer input shaft, to thereby prevent damage to said feedback transducer means such as could result from the exertion of excessive radially directed force upon said transducer input shaft.
 2. A servomechanism as defined in claim 1, wherein said angle is approximately 0*, whereby the combined forces exerted upon said transducer input gear by said drive gear and said driven gear at said first and second meshing regions produce a resultant force which is substantially exclusively circumferential whereby to exert upon said rotating transducer input shaft substantially zero radially directed force.
 3. A servomechanism as defined in claim 2, wherein said drive gear and said driven gear are coaxial.
 4. A servomechanism as defined in claim 2, wherein said drive gear and said driven gear are coaxial, of equal radius and identically toothed.
 5. A servomechanism as defined in claim 1, and further including biasing means for counteracting backlash in the transmission of force by said gears by permanently biasing said driven gear in one rotational direction, whereby when saId drive gear and said transducer input gear drive said driven gear in the opposite rotational direction the opposing torques applied to said transducer input gear by said drive gear and by said biasing means via said driven gear will combine to form a resultant force which is predominantly circumferential and does not have a substantial radial component.
 6. A servomechanism as defined in claim 3, wherein said angle is approximately 0*, whereby said resultant force will be substantially entirely circumferential and will have substantially no radial component.
 7. A servomechanism as defined in claim 6, wherein said drive gear and said driven gear are coaxial.
 8. A servomechanism as defined in claim 7, wherein said drive gear and said driven gear are coaxial, of equal radius and identically toothed.
 9. A servomechanism as defined in claim 2, and further including a first intermediate gear non-rotatably connected to and coaxial with said transducer input gear and driven by said drive motor, and a second intermediate gear non-rotatably connected to and coaxial with said driven gear and driving said controlled member.
 10. A servomechanism as defined in claim 6, and further including a first intermediate gear non-rotatably connected to and coaxial with said transducer input gear and driven by said drive motor, and a second intermediate gear non-rotatably connected to and coaxial with said driven gear and driving said controlled member. 